The distinctive feature of an electric stepping motor lies in the fact that it rotates in an angular movement representing a fraction of a revolution or step, this movement being carried out with high precision both in value of angle and in direction under the action of a drive pulse.
For example, a complete revolution requiring two hundred pulses will produce a step of 1.8.degree.. The number of steps per revolution depends on the number of phases of the motor, on its constructional arrangement and on the sequence of switching of the different phases.
The pulses are delivered to the system for controlling the motor windings in the form of a "frequency" signal, the frequency of which defines the speed of rotation in steps per second. A "direction" signal sets the switching of phases and therefore the direction of rotation of the motor.
When it is desired to start the motor from a zero initial velocity, if the control frequency at starting is too high, the motor is liable to start with a loss of step or may even not start at all. It is for this reason that a maximum starting frequency or so-called "start-stop" frequency has been defined.
The start-stop frequency is therefore the maximum frequency which permits starting and stopping without any loss of step. This frequency is a function of the motor itself but also of the inertia, of the resisting torque and of friction forces. Said frequency is therefore closely related to each particular use.
Stepping motors find applications in an increasingly wide range of different fields such as machine-tools, robotics, printing machines, drawing appliances, X-Y coordinate plotters, precision instruments for analysis or control.
In many of these applications, it is now desired to cause the stepping motor to carry out a given displacement in the shortest possible time. It is for this reason that, although it has been considered sufficient up to the present time and in the case of certain applications to operate the stepping motor at constant speed or in other words just below the start-stop speed defined earlier (usually substantially lower than 1000 steps per second, for example of the order of 300 steps per second in a given instance), it is now sought to accelerate the motor after startup to a speed which exceeds the start-stop value.
As will be readily apparent, a speed of rotation which is higher than the start-stop speed can be obtained after startup only by controlling the angular acceleration of the motor in order to prevent loss of synchronism. At each instant, the maximum angular acceleration which can be imparted to the motor is related to the driving torque, to the resisting torque and to the moment of inertia with respect to the drive shaft.
Attempts have therefore been made to develop control systems in which a given displacement is carried out automatically by the motor during a number N of steps over a minimum period of time.
A control operation under these conditions must be divided into three stages:
(a) starting at a speed below the start-stop speed followed by controlled acceleration to maximum speed; PA0 (b) operation at maximum speed; PA0 (c) controlled deceleration to a speed below the start-stop speed followed by stopping.
Rotation at speeds above the start-stop value permits a considerable reduction in times of positioning of a mechanism, particularly in the case of displacements of long duration. In fact, the maximum speed can be of the order of ten to twenty times higher than the start-stop speed (6000 to 7000 steps per second, for example).
In known types of control systems employed up to the present time, the law of speed increase from the starting speed V.sub.o to the maximum speed V.sub.m is linear in most cases in order to simplify the control. However, linear acceleration is quite obviously not an optimum law in regard to the motor characteristics (particularly in regard to the speed-torque characteristic of the motor) and does not permit optimum reduction of the time required for a given displacement.
It has also been proposed to adopt laws of speed increase in successive segments and in accordance with substantially parabolic or exponential curves. In all cases, however, the parameters of the law of speed increase (starting speed, maximum speed, time-duration of speed increase) cannot readily be programmed as a function of the particular conditions encountered. In consequence, the general practice consists in choosing a fixed value for these parameters and a fixed acceleration and deceleration program is introduced in a read-only memory (ROM) of the control system. In fact, programming very often entails the need for preliminary calculations and the programmed parameters are not independent with respect to each other. Finally, programming very frequently calls for other parameters which are directly related to the principle involved but do not directly characterize the movement to be performed.
Acceleration control systems are therefore attended by disadvantages either by reason of the fact that the acceleration curve is far from being of optimum shape or because the value of the various parameters to be taken into account is fixed or because the programming operation is both time-consuming and complicated while entailing the need for relatively powerful computing machines.
It is therefore difficult to obtain a minimum time-duration value for a given displacement of a mechanism with control systems of this type.
It has also been proposed in French Pat. No. 2,512,604, for example, to construct a stepping motor speed control device of the closed-loop type. In this case, however, it is necessary to provide a detecting element for producing synchronizing pulses which are representative of each incremental advance of the stepping motor.
The general objective of the present invention is to overcome the disadvantages of known systems of the first type described in the foregoing and to obtain optimum acceleration and deceleration curves without entailing the use of a detector device for producing synchronizing pulses as in the known systems of the second type described in the foregoing.
The system in accordance with the invention permits operation of the motor in accordance with an acceleration (and deceleration) curve of exponential shape having a negative exponent. As will hereinafter become apparent, this curve is the most favorable in the case of a stepping motor and permits simple processing of the different variable parameters to be introduced. This operation can be performed by means of a simple low-power microprocessor of a widely available 8-bit type, for example.